Process for producing hollow fibers having a non-uniform wall thickness and a non-uniform cross-sectional area

ABSTRACT

Hollow fibers, particularly useful as semipermeable membranes in separatory devices such as blood dialyzers, having a non-uniform cross-sectional area along the length thereof. The fibers have recurring points of maximum outside diameter and minimum outside diameter and such points may recur on a regular basis or at random. Fiber wall thicknesses may be substantially uniform or non-uniform. Methods and apparatus for making the fibers are disclosed.

This is a division, of application Ser. No. 19,973, filed Mar. 12, 1979,now U.S. Pat. No. 4,288,494 issued Sept. 8, 1981.

This invention relates generally to hollow fibers, and particularly tohollow fibers having a non-uniform cross-sectional area, that is, thecross-sectional area of the fibers varies along the length thereof. Evenmore particularly, the invention relates to cuproammonium cellulosehollow fibers having non-uniform cross-sectional areas and to methods ofmaking the same.

Currently, several types of hollow fibers are used as dialysis membranein artificial kidney dialyzers. One type of fiber is made of hydrolyzedcellulose acetate. These are made by acetylating cellulose to form thecellulose acetate, a thermoplastic polymer. The cellulose acetate issubsequently extruded, either as a polymer melt or as a solution in asuitable solvent, to form the desired hollow fiber. The celluloseacetate hollow fiber is thereafter subjected to chemical hydrolysis, forexample, in the presence of sodium hydroxide, to remove the acetategroups.

Another type of hollow fiber useful in dialysis is made from celluloseacetate. Such fibers are made as described above, but are not subjectedto chemical hydrolysis to remove the acetate groups. Cellulose acetateand cellulose triacetate hollow fibers for use in a permeabilityseparatory apparatus are disclosed in U.S. Pat. No. 3,228,877.

Still another type of hollow fiber which has been used in kidneydialysis is made from cuproammonium regenerated cellulose. These fibers,and a method for making them, are disclosed in U.S. Pat. No. 3,888,771.

The prior art hollow fibers heretofore used as dialysis membranes inartificial kidneys are generally circular in cross-section and have acontinuous lumen running the length thereof. They have a substantiallyuniform wall thickness and substantially constant inside and outsidediameters. As a result, the cross-sectional area of these fibers issubstantially constant.

It is well known that hollow fiber artificial kidneys comprise a bundleof semipermeable hollow fibers which are sealed at the ends thereof intoa suitable casing. The ends of the casing containing the hollow fibersare suitably manifolded, one manifold having a blood inlet, the othermanifold having a blood outlet. The casing itself has a dialysate inletand a dialysate outlet. In use, blood to be dialyzed flows internallythrough the cores of the hollow fibers, while the dialysate flowsexternally over the outer surfaces of the fibers. Impurities are removedfrom the blood by dialysis thereof through the walls of the hollowfibers. The impurities are dissolved in the flowing dialysate whichcarries them out of the dialyzer and the purified blood is returned tothe patient's body.

In accordance with the present invention, there are provided hollowfibers which have a non-uniform cross-sectional area, that is, thecross-sectional area of the fibers varies along the longitudinal axis ofthe fiber. The thickness of the wall of the fiber of the invention maybe uniform in both the longitudinal and transverse directions, or it mayvary longitudinally, or it may vary transversely, or it may vary bothlongitudinally and transversely. In general, the fibers of the inventionhave a circular cross-section but fibers having other cross-sectionalconfigurations, such as oval, can also be made.

The fibers of the invention have several advantages over the prior artfibers heretofore used in artificial kidney dialyzers and the likepermeability separatory devices. First of all, these fibers have a lowerbulk density than the above described prior art fibers. When thesefibers are gathered into a bundle, the resulting bundle has a lower bulkdensity than does a bundle of prior art fibers. In addition, the bundleof fibers, owing to the non-uniform cross-sectional area of the fiberstherein, has a certain degree of compliance and compressibility. Thus,after the hollow fibers of the invention are gathered into a bundle andthe bundle of fibers has been compressed and inserted into the casing ofa dialyzer or similar permeability separatory device, the fibers of thebundle will tend to expand laterally within the casing, thus producing awell dispersed fiber mass. Channeling of the externally flowing liquid,that is, the tendency of the externally flowing liquid to follow aparticular flow path through the permeability separatory device andpreferentially wash some fibers to the exclusion of others, is therebyreduced and a more uniform distribution of the externally flowing liquidis thereby promoted. Thus, for example, in a hollow fiber dialyzer, thedialysate is more uniformly distributed and the efficiency of dialysisis increased.

Another advantage of the hollow fibers of the invention is that, whenarranged in bundles, adjacent fibers tend to touch each other at alimited number of points along their lengths. The fibers thus act as"flow splitters," that is, the externally flowing liquid, when itreaches the point at which adjacent fibers come into contact with oneanother, is redirected into another flow pattern. Static mixing isthereby greatly improved as a result of which the concentration gradient(in the case of a mass transfer device) or the temperature gradient (inthe case of a heat exchange device) is desirably reduced.

Another, and very important, advantage associated with the non-uniformcross-sectional area hollow fibers of this invention is that mixing ofthe liquid flowing through the cores of the fibers is improved, thusimproving the efficiency of the mass transfer device. Referringspecifically to an artificial kidney dialyzer, the rate of blood flowthrough the hollow fibers is increased in those portions of the fiberwhich have relatively smaller cross-sectional areas and is decreased inthose portions of the fibers which have relatively largercross-sectional areas. The resulting pattern of acceleration anddeacceleration of the blood as it flows through the fibers set upsecondary flows in those portions of the fibers having the relativelylarger cross-sectional areas; these secondary flow patterns, in turn,tend to bring a fresh supply of blood to the internal surface of thefiber wall.

Fibers of the present invention are made with apparatus which includes aspinneret of the tube-in-orifice type. Such spinnerets are known in theart and have an inner chamber communicating with the outside of thespinneret housing. A hollow tube is secured within the aforementionedinner chamber. A fiber-forming material is supplied to the annular spacedefined by the outer surface of the tube and the wall of the innerchamber. Usually a fluid is supplied to the lumen of the hollow tube.

Hollow fibers having a non-uniform cross-sectional area and anon-uniform wall thickness are made by a process which includes thesteps of supplying a fiber forming material to the chamber of thespinneret, extruding the fiber forming material through the annularspace of the spinneret to form a hollow core extrudate, supplying afluid at a constant mean flow rate to the lumen of the hollow tube ofthe spinneret while periodically varying the pressure on said suppliedfluid, removing from the exit of the spinneret an extrudate containingthe fluid in the hollow core thereof; and converting the fiber formingmaterial to fiber form.

Hollow fibers having a non-uniform cross-sectional area and a uniformwall thickness are made by a process similar to that describedimmediately above, except that the fiber forming material is supplied tothe spinneret at a constant mean flow rate while periodically varyingthe pressure thereon.

BRIEF DESCRIPTION OF THE DRAWING

The invention will be better understood with reference to the appendeddrawings in which:

FIG. 1 is an enlarged longitudinal sectional view of one embodiment of ahollow fiber in accordance with the present invention;

FIG. 2 is a cross-sectional view taken along line 2--2 of FIG. 1;

FIG. 3 is a cross-sectional view taken along line 3--3 of FIG. 1;

FIG. 4 is a cross-sectional view taken along line 4--4 of FIG. 1;

FIG. 5 is a greatly enlarged view of a bundle of the hollow fibers ofFIG. 1, the bundle having empty spaces between the fibers;

FIG. 6 is a cross-sectional view taken along line 6--6 of FIG. 5;

FIG. 7 is a greatly enlarged longitudinal sectional view of the hollowfiber of FIG. 1 showing the secondary flow patterns set up within thefiber as a consequence of the non-uniform cross-sectional area of thefiber;

FIG. 8 is a schematic view, with certain parts shown in section, of anapparatus for making the hollow fibers of the invention;

FIG. 8A is a cross-section taken along line 8A--8A of FIG. 8; and

FIG. 9 is an enlarged longitudinal sectional view of another embodimentof a hollow fiber in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIGS. 1-4 of the drawing, there is shown a generallycircular hollow fiber 15 having a non-uniform cross-sectional area alongits longitudinal axis, represented by dashed line 16. In this particularembodiment, the thickness of the wall 18 of the fiber is generallyconstant both longitudinally, that is, as seen in FIG. 1, andtransversely, that is, as seen in FIGS. 2, 3 and 4. As illustrated inFIG. 2, fiber 15 has a maximum outside diameter, D₁, and, as illustratedin FIG. 3, a minimum outside diameter, D₂. As illustrated in FIG. 4, thefiber has an intermediate outside diameter, D₃. The cross-sectionalarea, CA₁, of fiber 15 at line 2--2 of FIG. 1 is equal to (D₁)² (π/4).The cross-sectional area, CA₂, of the fiber at line 3--3 of FIG. 1 isequal to (D₂)² (π/4), while the cross-sectional area, CA₃, of the fiberat line 4--4 of FIG. 1 is equal to (D₃)² (π/4). Cross-sectional area CA₁is larger than cross-sectional area CA₂. Cross-sectional area CA₃ offiber 15 is smaller than CA₁ and larger than CA₂. Thus, in the hollowfiber of the invention, the cross-sectional area of the fiber isnon-uniform, that is, it changes or varies, along its length. Intraveling along the length of the fiber in the left to right directionin FIG. 1, the cross-sectional area of the fiber has its maximum at line2--2, is smaller at line 4--4, and has its minimum at line 3--3.Continuing toward the right, the cross-sectional area increases until itagain reaches its maximum at point 20, after which, still continuing tothe right, the cross-sectional area decreases until it reaches itsminimum again at point 21. This variation in the cross-sectional areacontinues throughout the length of the fiber. The number of times inwhich points of maximum cross-sectional area and points of minimumcross-sectional area occur in a unit length of the fiber may be changedby making suitable adjustments in the manufacturing processes to bedescribed hereinafter.

The hollow fibers of the present invention can be made from a variety offiber forming polymers including the thermoplastic, fiber formingcellulose esters, such as cellulose acetate and cellulose triacetate,which, if desired, may be subsequently hydrolyzed to remove the estergroups. The hollow fibers may also be made from viscose regeneratedcellulose or cuproammonium regenerated cellulose, the latter beingparticularly desirable. Other fiber forming polymers such as, forexample, polyamides, polyesters, polycarbonates, polyacrylonitrile orpolyacrylonitrile copolymers, high density polyethylene, polypropyleneand polyvinyl alcohol, may also be used.

The term "fiber forming material" as used herein includes a polymer meltor a polymer solution. For example, nonuniform cross-sectional areahollow fibers of cellulose acetate, a thermoplastic polymer, may beformed by a process which includes extruding cellulose acetate in meltform through the orifice of a suitable tube-in-orifice spinneret,optionally stretching the extrudate, and allowing the extruded hollowfiber to solidify into its final form. As another example, it ispossible to use a process which includes the step of extruding asolution of the desired polymer in a solvent through the orifice of atube-in-orifice spinneret. In such a process, the solvent, if it issufficiently volatile, may be evaporated from the extruded polymersolution to produce the fiber in its final form. If a less volatilesolvent is used to dissolve the polymer, then the extruded polymersolution may be passed through a bath containing a material which willleach out the solvent used for the polymer solution but which will notdissolve the polymer material. In this case, the solvent will beextracted from the polymer solution, and the polymer will beprecipitated in its desired hollow fiber configuration.

Cuproammonium cellulose solution or viscose solution may be used toform, respectively, hollow fibers of cuproammonium regenerated celluloseor viscose. Cuproammonium regenerated cellulose hollow fibers, forexample, may be made by extruding a cuproammonium cellulose solutionthrough the orifice of a tube-in-orifice spinneret. After exiting thespinneret, the outside diameter of the extruded cuproammonium solutionmay be reduced by, for example, allowing the extrudate to fall freelythrough an open space for a predetermined distance. The extrudate isthen coagulated, regenerated, rinsed and dried before being wound up infinal form.

It will be understood by those skilled in the art that in the process offorming hollow fibers, the fiber forming material is extruded throughthe annular orifice of a tube-in-orifice spinneret. Depending on thedimensions of the spinneret and depending also on the fiber formingmaterial being used, it may or may not be necessary to supply a materialto the tube of the tube-in-orifice spinneret. For example, when athermoplastic polymer like cellulose acetate is used in melt form as thefiber forming material, it may not be necessary to supply any materialto the tube. In other cases, for example where it is desired to makehollow fibers from cuproammonium solution or viscose solution, it may benecessary to supply a material in liquid or gaseous form to the tube ofthe tube-in-orifice spinneret. When this is done, the core of theextruded fiber forming material contains the liquid or gas which in turnprevents collapse of the hollow fiber structure. Gas, e.g., air ornitrogen, supplied under appropriate pressure, will prevent collapse ofthe extruded material; a liquid will likewise prevent undesiredcollapse.

In the case of cuproammonium cellulose solution, the liquid supplied tothe core of the extrudate may be removed at any time after the extrudedcuproammonium cellulose has been regenerated to produce the fiber.

A tube-in-orifice spinneret useful for producing fibers in accordancewith the invention is disclosed in U.S. Pat. No. 3,888,771. Theextrusion apparatus disclosed in British Pat. No. 534,618 may also beused.

As indicated earlier herein, the non-uniform cross-sectional area fibersof the present invention are produced by a process which includes

(i) the step of supplying the fiber forming material to the annularorifice of the spinneret at a constant mean flow rate while varying thepressure thereon; or

(ii) the step of supplying a core material to the hollow tube of thetube-in-orifice spinneret at a constant mean flow rate while varying thepressure applied thereto; or

(iii) the step of supplying both the fiber forming material and the corematerial to the spinneret at a constant mean flow rate while varying thepressure applied to each of said materials.

Referring to FIG. 8, there is illustrated apparatus 28 for making hollowfibers according to the present invention. Spinneret 30 comprises ahousing 31 having a circular chamber 33 defined by interior wall 33a.One end of chamber 33 communicates with the outer bottom surface of thespinneret, while its other end communicates with inlet 34 in the side ofthe housing. Circular tube 32 has a lumen 32a extending the lengththereof and is concentrically fixed within chamber 33. One end of tube32 communicates with the outer bottom surface of the spinneret, whilethe other end conveniently projects somewhat beyond the top surface ofthe spinneret. The outer surface of tube 32 cooperates with interiorwall 33a to define an annular space 35 through which fiber formingmaterial is extruded. A core fluid, either a liquid or a gas, may bepassed through lumen 32a of tube 32.

Apparatus 28 further comprises a first reservoir 36, a first pump means38, and means 40 for varying the pressure of the core fluid beingsupplied to the interior of hollow tube 32. Reservoir 36, pump means 38,and fluid pressure varying means 40 are connected as illustrated in FIG.8 by suitable conduit material 42. A supply of core fluid 44 is held inreservoir 36. From there, fluid 44 passes through pump means 38, thencethrough fluid pressure-varying means 40, and finally into the interiorof tube 32.

Apparatus 28 also comprises a second reservoir 76, a second pump means78, and means 80 for varying the pressure of the fiber forming materialbeing supplied through inlet 34 to chamber 33 of the spinneret.Reservoir 76, which holds fiber forming material 84, pump means 78, andpressure varying means 80 are connected as illustrated at the left handside of FIG. 8 by conduit means 42. It will be noticed that apparatus 28has a pair of two-way valves 43, 43' one on the upstream side of means80, the other on the downstream side. These are connected by conduitmeans 42'. Thus, depending on the setting of the two-way valves, fiberforming material 84 can be pumped either through pressure varying means80 or it may bypass means 80 and be pumped directly to inlet 34 of thespinneret.

The extrudate 50 exiting spinneret 30 comprises a wall 52 of fiberforming material 84 surrounding core fluid 44 (illustrated by stipplingin the lower central portion of FIG. 8). The extrudate, aftersolidifying (in the case the fiber forming material is a polymer melt)or after removal of solvent (in the case where the fiber formingmaterial is a polymer solution) or after coagulation and regeneration(in the case where the fiber forming material is cuproammonium cellulosesolution or viscose solution) has the non-uniform cross-sectional areastructure described earlier herein in connection with FIGS. 1-4. If thecore fluid is a gas such as air, then it need not be removed from theinterior of the fiber. If the core fluid is a liquid, such liquid can beremoved at any time after the desired fiber has been formed.

The non-uniform cross-sectional area of the fiber of the invention isproduced by the action of pressure varying means 40 on the core fluid44. The outer diameter of the extrudate is varied by varying thepressure of the core fluid. The higher the pressure of the core fluid atthe exit of spinneret 30, the larger will be the outside diameter ofextrudate 50 at that point. Lower pressure gives correspondingly smallerouter diameters. Thus, varying the pressure on the core fluid suppliedto annular space 35 produces an extrudate 50, and ultimately a hollowfiber, having a non-uniform cross-sectional area.

Means 40 for varying the pressure on core fluid 44 may take severaldifferent forms. As illustrated in FIG. 8, core fluid pressure varyingmeans 40 comprises an enclosed oscillatory chamber 60 having an inletand outlet and defined at least in part by a flexible diaphragm 62. Thepressure in chamber 60 is controlled by electromagnetic coil 64 actingon diaphragm 62. Coil 64 is driven by an electronic oscillator 65,similar to an audio oscillator. Oscillator 65 is such that the waveform, wave amplitude, and wave frequency of the electrical energysupplied to coil 64 can be varied. Various wave forms, e.g., sine,triangular or random, can be employed. Thus, in use, the electricalenergy supplied by electronic oscillator 65 is transmitted throughelectromagnetic coil 64 to diaphragm 62. The diaphragm, responsive tothe energy supplied to it by the oscillator and coil, alternatelycompresses and rarifies core fluid 44 flowing through chamber 60. Themotion of the diaphragm varies the pressure on the core fluid, thusproducing an alternating pressure at the exit of spinneret 30. This, inturn, produces the variation in cross-sectional area of the extrudate50.

EXAMPLE I

A cuproammonium regenerated cellulose hollow fiber having a non-uniformwall thickness and a non-uniform cross-sectional area is made usingapparatus 28 shown in FIG. 8. Tube 32 of the spinneret has an outsidediameter of 0.040 inch and an inside diameter of 0.015 inch. The outerdiameter of annular orifice 35 at the exit of the spinneret is 0.190inch, i.e., the distance between the outer surface of tube 32 and innerwall 33a is 0.075 inch.

The core fluid, i.e., the material to be supplied to lumen 32a of hollowtube 32 and which will be contained in the hollow core of extrudate 50exiting the spinneret, is isopropyl myristate, a supply of which isstored in reservoir 36.

Pump 38 is a non-pulsatile, positive displacement metering pump which iscapable of supplying the isopropyl myristate core liquid at a selectedconstant mean flow rate to oscillatory chamber 60. Diaphragm 62 is asheet of polyester film, 3 mils thick. Electronic oscillator 65 has atriangular wave output with power amplifier for driving the externalelectromagnetic coil 64 at approximately 8 watts with adjustable outputamplitude. The electronic oscillator is set to operate at 750 cycles perminute.

The fiber forming material is a conventional cupro-ammonium cellulosesolution containing 8.5% by weight cellulose (purified cotton linters),3.5% by weight copper (as Cu⁺⁺) and 18.2% by weight ammonium (NH₄ ⁺).The viscosity of the cuproammonium cellulose may conveniently range fromabout 25000 to 55000 centipoises at 25° C. and its specific gravity is1.3 at 25° C. A supply of the cuproammonium cellulose solution is heldin reservoir 76. Valves 43, 43' are set so that the cuproammoniumcellulose solution bypasses oscillatory chamber 80, that is, thecuproammonium cellulose solution is pumped by non-pulsatile, positivedisplacement metering pump 78 through by-pass conduit 42' to inlet 34 ofspinneret 30.

The lines connecting reservoir 36, pump 38, oscillatory chamber 44 andthe inside of hollow tube 32 are purged with the isopropyl myristatecore material. The lines connecting reservoir 76, pump 78 and inlet 34are purged with the cuproammonium cellulose solution.

To produce the desired cuproammonium hollow fiber, the aforementionedcuproammonium cellulose solution is supplied to spinneret 30 at a rateof about 6 grams (4.6 ml) per minute. As can be seen in FIG. 8, thecuproammonium cellulose solution first flows through inlet 34, thenthrough annular space 35 defined by the outer surface of tube 32 and thewalls of chamber 33, after which it leaves the spinneret exit asextrudate 50.

As the cuproammonium cellulose solution is being supplied to inlet 34 ofspinneret 30, the isopropyl myristate core liquid is supplied to thelumen of hollow tube 32 at a constant mean flow rate of about 3.2grams/minute. The core liquid enters the hollow tube at the top of thespinneret and leaves at the bottom. The pressure on the core liquid isuniformly varied by the compressing and rarifying action of diaphragm 62responding to the action of electromagnetic coil 64 which is driven byoscillator 65 operating at 750 cycles/minutes. As the core liquid leavesthe spinneret, it is enclosed in the hollow core of the extrudedcuproammonium cellulose solution. The mean pressure applied to thesystem depends on several factors such as, for example, the dimensionsof the tube of the spinneret and the dimensions of oscillatory chamber.In general, the instantaneous pressure on the core liquid ranges from apeak value of about three times the mean pressure to a minimum value ofabout one-third of the mean pressure.

The extrudate 50 is allowed to fall through an open space of about 11cm. into a coagulation bath of 15% NaOH. This distance may be varied inorder to control the outer diameter of the final fiber. Sodium hydroxidecoagulation baths are well known in the cuproammonium cellulose fiberart; it is also well known that the concentration of sodium hydroxidemay be varied if desired. The coagulated extrudate is led from thecoagulating bath into a regenerating bath, after which it is rinsed;plasticized or otherwise treated, if desired; dried; and collected on asuitable collection device. The regenerating bath is 3% by weightaqueous sulfuric acid. The coagulated and regenerated fiber is rinsed ina water bath, dried at 110° C., and collected. It will be recognized bythose skilled in the art that other coagulating and regenerating bathswell known in the art may be used in place of the sodium hydroxidecoagulating bath and the sulfuric acid regenerating bath employed inthis Example I.

The resulting fiber 90 is illustrated in longitudinal section in FIG. 9.The fiber has a maximum outside diameter at point 91 of about 240microns. The wall thickness at the point of maximum outside diameter isabout 9 microns. The fiber has a minor diameter at point 93 of about 175microns. The wall thickness at the point of minor diameter is about 13microns. The number of points of maximum outside diameter in fiber 90may range from about one/meter to about two hundred/meter, andpreferably from about five/meter to about fifty/meter. In fiber 90,which is made by regularly varying the pressure applied to the coreliquid, the number of points of minimum outside diameter wouldcorrespond to the number of points of maximum outside diameter. Thus thetotal number of points of maximum outside diameter and minimum outsidediameter may range from about 2/meter to about 400/meter, and preferablyfrom about 10/meter to about 100/meter. The fiber wall thickness isgreatest at the point of minimum fiber diameter and smallest at thepoint of maximum fiber diameter. Fiber 90 is semipermeable and is usefulin a variety of permeability separatory devices. In particular, it isuseful as a semipermeable hollow fiber membrane for artificial kidneydialyzers.

FIG. 6 illustrates a bundle 100 of hollow fibers 90 as same might appearwhen included in a blood dialyzer. Void spaces 95 are present betweenthe fibers, such void spaces acting as the "flow-splitters" referred tohereinabove.

In the method of this Example I, the pressure applied to the core liquidvaries on a regular basis, i.e., the peak and minimum pressure valuesrecur at fixed or uniform time intervals. This regularity results in ahollow fiber 90 which has the following characteristics: (a) the spacingbetween any point 91 of maximum outside diameter and the next adjacentpoint of maximum outside diameter is substantially constant along thelength of the fiber; (b) the spacing between any point 93 of minimumoutside diameter and the next adjacent point of minimum outside diameteris substantially constant along the length of the fiber; and (c) thespacing between any point of maximum outside diameter and the nextadjacent point of minimum outside diameter is substantially constantalong the length of the fiber. In other words, the points of majoroutside diameter and the points of minimum outside diameter haveconstant repeat distances.

It will be recognized, however, that the pressure applied to the corefluid may also be varied on a random basis, i.e., the peak and minimumpressures may recur at non-fixed or non-uniform time intervals. In sucha case, the points of maximum and minimum outside diameters will occurat random along the length of the fiber, that is, the points of maximumoutside diameter and the points of minimum outside diameter will haverandom repeat distances. The total number of points of maximum outsidediameter and minimum outside diameter may range up to four hundred/meteror even more. Hollow fibers having such randomly occurring points ofmaximum and minimum outside diameters are particularly useful as hollowfiber membrances in blood dialyzers and other semipermeable separatorydevices because, in addition to having the "flow-splitting"characteristics mentioned earlier herein, interdigitation of the bundledfibers is greatly reduced and undesirable channeling affects areminimized.

It will be understood that the spacings referred to above can be changedby changing the frequency with which the pressure in the core fluid isvaried. The higher the frequency at which the pressure is varied, thecloser will be the spacing between a point of maximum diameter and apoint of minimum diameter. Similarly, reducing the frequency at whichthe pressure on the core fluid is varied will reduce such spacing.

EXAMPLE II

A cuproammonium regenerated cellulose hollow fiber having asubstantially uniform wall thickness and a non-uniform cross-sectionalarea is made using the apparatus of FIG. 8. In this case, however, thetwo way valves 43, 43' are set so that the cuproammonium cellulosesolution is pumped first through means 80 for varying the pressureapplied to said solution and then to inlet 34 of the spinneret.

Means 80 for varying the pressure applied to the fiber forming materialcorresponds generally to means 40 for varying the pressure applied tothe core fluid. Thus, means 80 comprises an oscillatory chamber 60'having a flexible diaphragm 62', an electromagnetic coil 64' which actson the diaphragm and an electronic oscillator 65'.

The core liquid is isopropyl myristate and the fiber forming material isthe same cuproammonium cellulose solution described in Example I above.

The process of this Example II is similar to that of Example I exceptthe Example II process includes the additional step of uniformly varyingthe pressure on the cuproammonium cellulose solution. Thus, in order toproduce a hollow fiber having a substantially uniform wall thickness anda non-uniform cross-sectional area, it is necessary to (1) supply thecore liquid at a constant mean flow rate and under a varying pressure tothe lumen of hollow tube 32 of the spinneret, and (2) supply thecuproammonium cellulose solution at a constant mean flow rate and undera varying pressure to the inlet 34 of the spinneret from whence it flowsthrough, and out of, the annular orifice 35 of the spinneret. Thepressure applied to the fiber forming material and the core liquid mustbe the same; hence, it is necessary to operate oscillator 65, 65' inexact synchronization.

Specifically, the isopropyl myristate core liquid is supplied at aconstant mean flow rate of 3.2 grams per minute; simultaneously, thepressure on the isopropyl myristate is uniformly varied by theaforementioned action of pressure varying means 40 in which oscillator65 operates at a frequency of about 750 cycles/minute.

At the same time the isopropyl myristate is being supplied in theabove-described manner to hollow tube 32, the cuproammonium cellulosesolution is supplied to inlet 34 at a constant mean flow rate of about 6grams (4.6 mls) per minute. The pressure on the cuproammonium cellulosesolution is uniformly varied by pressure varying means 80 in whichoscillator 65' operates at a frequency of 750 cycles/minute. Oscillators65 and 65' are in phase and are exactly synchronized. The extrudateexiting the spinneret comprises a wall of the cuproammonium cellulosesolution in which is contained the core liquid isopropyl myristate. Theextruded material is allowed to fall into a coagulating bath, afterwhich it is regenerated, rinsed, dried, and collected. The coagulating,regenerating, rinsing, drying and collecting steps are carried out asdescribed in Example I.

The fiber produced by the process of this Example II is shown inlongitudinal section in FIG. 1. The fiber has a substantially uniformwall thickness of approximately 11 microns. The fiber has a maximumoutside diameter, D₁, of about 240 microns and a minimum outsidediameter, D₂, of about 175 microns. Hollow fibers produced by theabove-described process of Example II are semipermeable, have asubstantially uniform wall thickness, have a non-uniform cross-sectionalarea, have substantially constant repeat distances, and are useful in avariety of permeability separatory devices. In particular, such fibersare useful as semipermeable hollow fiber membranes for artificial kidneydialyzers. In the fiber of Example II, which is made by regularlyvarying the pressure applied to the core liquid and the fiber formingmaterial, the number of points of minimum outside diameter wouldcorrespond to the number of points of maximum outside diameter. Thus thetotal number of points of maximum outside diameter and minimum outsidediameter may range from about 2/meter to about 400/meter, and preferablyfrom about 10/meter to about 100/meter.

It will be recognized that uniform wall thickness, non-uniformcross-sectional area hollow fibers having random repeat distances can bemade by the process of Example II if the pressure applied to the corematerial and the fiber forming material is varied on a random but inphase and exactly synchronized basis. In such random fibers, the totalnumber of points of maximum outside diameter and minimum outsidediameter may range up to four hundred/meter or even more.

MODIFICATIONS

Various modifications may be made in practicing the methods of thepresent invention. Electronic oscillators 65, 65' may be operated inorder to vary the pressure on the core liquid and on the fiber formingmaterial, respectively, at frequencies in the range of from about 50cycles/minute to about 10,000 cycles/minute, preferably at frequenciesranging from about 500 cycles/minute to about 5000 cycles/minute.

The size of the fiber may be varied by varying the dimensions of thespinneret; by varying the distance through which the extrudate fallsafter exiting the spinneret and before it reaches its coagulating bathor is otherwise set or gelled; by varying the relative flow rate of thecore liquid and/or the fiber forming material; and (as indicated above)by varying the frequency with which the pressure on the core fluid orfiber forming material is changed.

Hollow fibers may be made in a wide range of maximum outside diametersand minimum outside diameters. Hollow fibers particularly useful inpermeability separatory devices such as blood dialyzers may have maximumoutside diameters ranging from about 20 microns to several hundred oreven a thousand microns; preferably, the maximum outside diameters ofsuch fibers range from about 150 microns to about 300 microns. Suchhollow fibers may have minimum outside diameters ranging from about 0.9to about 0.25 times the maximum outside diameter and preferably fromabout 0.8 to about 0.3 times the maximum outside diameter.

In the case of hollow fibers having a substantially uniform wallthickness, the wall thickness may range from 2 microns to about 80microns. Wall thicknesses in the range of 2 microns to about 20 micronsare even more preferred.

In the case of hollow fibers which have non-uniform wall thicknesses,the mean wall thickness may be in the 8 micron to 80 micron range, andpreferably in the 8 micron to 20 micron range. The ratio of the minimumwall thickness to the maximum wall thickness, ^(t) min/^(t) max, shouldbe in the range of 0.25-0.9, preferably in the range of 0.3-0.8.

We claim:
 1. A method for producing a regenerated cellulose hollow fiberhaving a hollow core extending continuously throughout the lengththereof, said fiber having a non-uniform wall thickness and anon-uniform cross-sectional area, said method comprising(a) providing atube-in-orifice spinneret comprising a housing having a chambercommunicating with an outer surface thereof and a hollow tube having acontinuous lumen extending the length thereof secured within saidchamber, the walls of said chamber and the outer surface of said tubedefining an annular space through which a fiber forming material may beextruded; (b) supplying to said chamber a fiber forming materialselected from the group consisting of cuprammonium cellulose solutionand viscose cellulose solution; (c) extruding said fiber formingmaterial through said annular space to form a hollow core extrudate; (d)supplying a fluid at a constant mean flow rate to the lumen of saidhollow tube while simultaneously varying the pressure on said suppliedfluid; (e) removing from the exit of said spinneret said extrudate withsaid fluid contained in the hollow core thereof; and (f) converting saidfiber forming material to fiber form.
 2. A method according to claim 1wherein the instantaneous pressure on said supplied fluid has a peakvalue of about three times the mean pressure and a minimum value ofabout one-third of the mean pressure.
 3. A method according to claim 1wherein the pressure on said supplied fluid is varied on a regularbasis.
 4. A method according to claim 1 wherein the pressure on saidsupplied fluid is varied on a random basis.
 5. A method according toclaim 1 wherein the pressure on said supplied fluid is varied at afrequency within the range of about 50 cycles/minute to about 10,000cycles/minute.
 6. A method according to claim 1 wherein the pressure onsaid supplied fluid is varied at a frequency within the range of about500 cycles/minute to about 5,000 cycles/minute.
 7. A method according toany one of claims 1 and 2 through 6 inclusive wherein said fiber formingmaterial is cuprammonium cellulose solution.
 8. A method according toany one of claims 1 and 2 through 6 inclusive wherein said fiber formingmaterial is cuprammonium cellulose and said supplied fluid is isopropylmyristate.
 9. A method according to any one of claims 1 and 2 through 6inclusive wherein said fiber forming material is viscose cellulosesolution.
 10. A method for producing a cuproammonium regeneratedcellulose hollow fiber having a hollow core extending continuouslythroughout the length thereof, said fiber having a non-uniform wallthickness and a non-uniform cross-sectional area, said methodcomprising(a) providing a tube-in-orifice spinneret comprising a housinghaving a chamber communicating with an outer surface thereof and ahollow tube having a continuous lumen extending the length thereofsecured within said chamber, the walls of said chamber and the outersurface of said tube defining an annular space through which a fiberforming material may be extruded; (b) supplying a cuproammoniumcellulose solution to said chamber; (c) extruding said cuproammoniumcellulose solution through said annular space to form a hollow coreextrudate; (d) supplying isopropyl myristate at a constant mean flowrate to the lumen of said hollow tube while simultaneously varying thepressure on said supplied fluid, said pressure having a peak value ofabout 3 times the mean pressure and a minimum pressure of about 1/3 ofthe mean pressure; (e) removing from the exit of said spinneret saidextrudate with said fluid contained in the hollow core thereof; and (f)converting said cuproammonium cellulose solution to fiber form.
 11. Amethod according to claim 10 wherein the conversion of the extrudedcuproammonium cellulose solution to fiber form is achieved bycoagulating said extrudate, regenerating said coagulated extrudate, andrinsing said coagulated and regenerated extrudate.